1. Field of the Invention
The present invention relates to the technical field of touch panels and, more particularly, to an in-cell multi-touch panel system with low noise and time division multiplexing and its driving method.
2. Description of Related Art
The principle of touch panels is based on different sensing manners to detect a voltage, current, acoustic wave, or infrared to thereby detect the coordinates of touch points on a screen where a finger or other medium touches. For example, a resistive touch panel uses a voltage difference between the upper and lower electrodes to compute the position of a pressed point for detecting the location of the touch point, and a capacitive touch panel uses a capacitance change generated in an electrostatic combination of the arranged transparent electrodes and a human body to generate a current or voltage for detecting touching coordinates.
Upon the principle, the capacitive touch technologies can be divided into a surface capacitive touch sensing and a projected capacitive touch sensing. The surface capacitive touch sensing has a simple configuration, so that the multi-touch implementation is not easy, and the problems of electromagnetic disturbance (EMI) and noises are difficult to be overcome. Therefore, the popular trend of capacitive touch development is toward the projected capacitive sensing.
The projected capacitive touch sensing can be divided into a self capacitance and a mutual capacitance sensing. The self capacitance sensing indicates that a capacitance coupling is generated between a touch object and a conductor line, and a touch occurrence is decided by measuring a capacitance change of the conductor line. The mutual capacitance sensing indicates that a capacitance coupling is generated between two adjacent conductor lines when a touch occurs.
A typical self capacitance sensing senses the grounded capacitance (Cs) on every conductor line. Thus, a change of the grounded capacitance is used to determine whether an object approaches to the capacitive touch panel. The self capacitance or the grounded capacitance is not a physical capacitor, but parasitic and stray capacitance on every conductor line. FIG. 1 is a schematic view of a typical self capacitance sensing. As shown in FIG. 1, at the first period, the driving and sensing devices 110 in a first direction drive the conductor lines in the first direction in order to charge the self capacitance (Cs) of the conductor lines in the first direction. At the second period, the driving and sensing devices 110 sense the voltages on the conductor lines in the first direction to thereby obtain m data. At the third period, the driving and sensing devices 120 in a second direction drive the conductor lines in the second direction in order to charge the self capacitance of the conductor lines in the second direction. At the fourth period, the driving and sensing devices 120 sense the voltages on the conductor lines in the second direction to thereby obtain n data. Therefore, a total of m+n data can be obtained.
The typical self capacitance sensing of FIG. 1 connects both a driver circuit and a sensor circuit on the same conductor line in order to drive the conductor line and sense a signal change on the same conductor line to thereby decide the magnitude of the self capacitance.
Another way of driving the typical capacitive touch panel is to sense the magnitude change of mutual capacitance Cm to thereby determine whether an object approaches to the touch panel. Similarly, the mutual capacitance Cm is not a physical capacitor but a mutual capacitance between the conductor lines in the first direction and in the second direction. FIG. 2 is a schematic diagram of a typical mutual capacitance sensing. As shown in FIG. 2, the drivers 210 are located on the first direction (Y), and the sensors 220 are located on the second direction (X). At the upper half of the first period T1, the drivers 210 drive the conductor lines 230 in the first direction and use the voltage Vy_1 to charge the mutual capacitance (Cm) 250, and at the lower half, all sensors 220 sense voltages (Vo_1, Vo_2, . . . , Vo_n) on the conductor lines 240 in the second direction to thereby obtain n data. Accordingly, the m×n data can be obtained after m driving periods.
A typical flat touch display is produced by stacking a touch panel directly over a flat display. Since the stacked touch panel is transparent, the image on the flat display can be displayed by passing through the stacked touch panel, and the touch panel can act as an input medium or interface.
However, such a stacking requires an increase of the weight of the touch panel, resulting in relatively increasing the weight of the flat display, which cannot meet with the compact requirement for current markets. For a further description, when the touch panel and flat display are stacked directly, the increased thickness reduces the transmittance of rays and increases the reflectivity and haziness, resulting in relatively reducing the display quality on the screen.
To overcome this, the embedded touch control technology is adapted. The currently developed embedded touch control technologies are essentially on-cell and in-cell technologies. The on-cell technology uses a projected capacitive touch technology to form sensors on the backside (i.e., a surface for attaching a polarized plate) of a color filter (CF) for being integrated into a color filter structure. The in-cell technology embeds sensors in an LCD cell to thereby integrate a touch element with a display panel such that the display panel itself is provided with a touch function without having to be attached or assembled to a touch panel. Such a technology typically is developed by a TFT LCD panel factory. The in-cell multi-touch panel technology is getting more and more mature, and since the touch function is directly integrated during a panel production process, without adding a layer of touch glass, the original thickness is maintained and the cost is reduced.
FIG. 3 is a schematic view of a configuration of a typical in-cell multi-touch panel 300. In FIG. 3, the panel 300 includes a lower polarizer 310, a lower glass substrate 320, a thin film transistor (TFT) or LTPS layer 330, a liquid crystal (LC) layer 340, a common voltage and touch driving layer 350, a color filter layer 360, an upper glass substrate 370, a sensing electrode layer 380, and an upper polarizer 390. As shown in FIG. 3, in order to save the cost, a touch sensor is integrated with an LCD panel, and the common voltage layer (Vcom) of the LCD panel is located at a layer as same as the drivers of the touch sensor, thereby forming the common voltage and touch driving layer 350. Thus, the cost saving is achieved. The sensing electrode layer 380 is located on the upper glass substrate 370. The TFT or LTPS layer 330 is constructed of thin film transistors (TFTs) or low-temperature poly-Si film transistors (LTPS) 332 and transparent electrodes 331.
As cited, the self capacitive sensing and the mutual capacitive sensing make use of the driver lines in a touch IC to input driving signals, and the sensing circuit can collect different charge generating voltage signals Vo_1-Vo_n from a capacitance change, so as to determine whether an object approaches to or touches touch sensors based on the signal change.
FIG. 4 is a schematic view of a capacitance of a typical in-cell touch display panel, where CLC indicates a capacitance between a thin film transistor (TFT) and a common voltage layer (Vcom), Cparasitism 1 indicates a capacitance between a sensing line of a touch integrated circuit (IC) and the TFT, and Cparasitism 2 indicates a capacitance between the sensing line of the touch IC and the common voltage layer Vcom. However, the panel is subjected to noises originated from various sources, such as noises generated by a source voltage of the TFT of the panel and the polarity inversion of the panel, which may seriously interfere the sensing lines of the touch IC and make a touch device to generate an erroneous coordinate determination and an instable problem. Thus, the system signal to noise ratio SNR is relatively reduced.
Therefore, the prior art typically adds digital and analog filters in the touch IC to eliminate the affection to the touch IC caused by the displaying and driving IC on the panel. However, the anti-noise capability of a filter is varied with different noise sources, so that the noise interference from the displaying and driving IC to the touch IC may not be effectively overcome.
In addition, the configuration of the in-cell multi-touch panel in FIG. 3 uses a time sharing to divide one frame into a display cycle and a touch cycle to thereby share the common voltage layer Vcom of the panel and the driving layer of the touch sensor, as shown in the timings of FIGS. 5(A) and 5(B).
As shown in FIG. 5(A), the time for one display frame is divided into one display cycle and one touch cycle, and the frame of the display panel is displayed in the display cycle before the touch sensing is performed in the touch cycle. In US 2012/0050217 entitled “Display device with touch detection function, control circuit, driving method of display device with touch detection function, and electronic unit”, the timing of a first embodiment (shown in FIG. 8 of the patent publication) uses the same strategy as that in FIG. 5(A); i.e., the frame is displayed before the touch sensing is performed. Another time sharing is illustrated in FIG. 5(B), which divides the time for one line into a display cycle and a touch cycle, while performing a frame display on the panel first and then a touch sensing.
However, such a time sharing needs a driving IC to drive more and more pixels as a resolution of the panel becomes higher and higher, resulting in requiring longer and longer time. However, because the display frame rate has to be maintained at 60 Hz and over, i.e., 16.6 ms for each frame, it becomes difficult to perform image displaying and touch sensing in 16.6 ms since the resolution of the panel becomes higher and higher. Also, the resolution development is further limited, which becomes a serious problem to be solved. In addition, the electrical field has to continuously alternate the positive and negative polarities for the liquid crystals, so that the noise is relatively high in the panel, resulting in that a touch sensing circuit may make an erroneous determination and generate an instable touch coordinate.
Therefore, it is desirable to provide an improved in-cell multi-touch panel system to mitigate and/or obviate the aforementioned problems.